Physica Medica: European Journal of Medical Physics - Volume 32 - Supplement 1 - February 2016

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used at the Catana protontherapy passive facility for the treatment of uveal
melanoma (62 MeV). Narrow shaped beams, with field area ranging from
30 to 300 mm2 , are involved, corresponding to effective collimator diam-
eters of 6–20 mm.
PP Markus ion chamber, with a 6 mm cavity diameter, has been adopted
as a reference dosimeter for modulated proton beams, but if the field size
is smaller than twice the diameter of the cavity, a detector with a better
spatial resolution must be used. For this purpose, the PTW T60020 dosim-
etry diode PR, intended for dose measurements in small proton fields, was
investigated.
Materials and method: The PTW detector is a disk-shaped Hi-pSi silicon
waterproof diode (p-type Silicon, high-doped) with a very small active
volume (0.02 mm3 , r = 0.56 mm, thickness = 20 μm). Despite the small size
of active volume, the nominal response (nC/Gy) is about ten times larger
than PTW pin-point chambers.
Results: The diode detector was preliminary characterized in terms of short
term precision, linearity and energy dependence, using dose rates typical
of eye protontherapy (10.30 Gy/min).
As regard depth-dose curves, the shape of Bragg curve correlates with those
measured with PP chamber in terms of beam range, distal dose fall-off, en-
trance dose and SOBP length for modulated beams.
The excellent transverse spatial resolution allows measuring very precise-
ly beam profiles even in the penumbra region of narrow fields, more
accurately than pin-point chambers.
The diode can be used for determining output factor (OF, cGy/UM) up to
collimator 5 mm in diameter, as a result of an intercomparison with
radiochromic EBT3 films.
Conclusions: Precise definition of measurement point and high spatial res-
olution parallel and perpendicular to beam direction allows very accurate
measurements of depth-dose curves, lateral profiles and OF.
A.189
EARLY PREDICTION OF INDIVIDUAL RESPONSE IN NEO-ADJUVANT
ADAPTIVE RADIOCHEMOTHERAPY FOR RECTAL CANCER
R. Raso P. Passoni A. Palmisano C. Fiorino G.M. Cattaneo
F. De Cobelli A. Esposito P. Mangili N. Slim N.G. Di Muzio
R. Calandrino a Medical Physics Department, San Raffaele Scientific Institute,
Milano, Italy; b Radiotherapy Department, San Raffaele Scientific Institute,
Milano, Italy; c Radiology Department, San Raffaele Scientific Institute, Milano,
Italy
Introduction: Developing a radiobiological model predicting individual
outcome for rectal cancer patients (RCPs) treated with an adaptive con-
comitant boost (ACB) during neo-adjuvant radiochemotherapy (RCH).
Material and methods: Forty-two RCPs were treated with helical
tomotherapy (18 × 2.3 Gy) delivering an ACB of 3 Gy/fr on the residual
gross tumor volume (GTV) in the last 6 fractions (fr). GTVs were con-
toured by a single radiologist on axial T2 MRIs acquired for initial
planning (Vpre), at 9 fr for the ACB planning (Vm) and before surgery
(Vp). The Poisson-like parameter (1 − dV(D))^Vpre was chosen as surro-
gate for TCP, being dV(D) = Vm/Vpre or Vp/Vpre, with D at 9fr (TCPm) or
at end RCH (TCPp). The predicting power of TCPm/p for pathological
complete remission (pCR, n = 14) was assessed by the ROC curve AUCs.
Then, two-variable logistic (LOGm, LOGp) models including Vpre and
dV(D) were considered. ROC curves of the 4 models were compared. The
correlation between the models and an estimate of the residual cells at
surgery (Vs = fraction of viable cells in the surgical specimen × Vp) was
evaluated with the Spearman test.
Results: All models showed a high discriminative power for pCR (p < 0.0001).
AUC for TCPmwas 0.87 (specificity: 71.4%, sensitivity: 96.4%, cut-off: 0.0012),
higher than TCPp (0.82). TCPm/p were highly correlated with Vs (R = 0.77/
0.74, p < 0.0001). Similar results were found for LOGm/p (AUC = 0.90/
0.87; R = 0.79/0.77) without significant differences between TCP and LOG
models.
Conclusion: A radiobiologically consistent model including early regres-
sion (TCPm) measured on T2-MRI images well predicts pCR and is strongly
correlated with the estimated residual cell number after adaptive RCH;
similar performances were obtained with a logistic model including Vpre
and Vm/Vpre. The corresponding models using Vp showed a slightly worse
discriminative power. Therefore MRI volumes measured before and during
RCH have a great potential to better individualize adaptive RCH.
A.190
A COMPARISON BETWEEN STATIC AND HELICAL CALIBRATION OF THE
NEW OCTAVIUS DETECTOR 729 IN TOMOTHERAPY
V. Ravaglia *, M. Quattrocchi, S. Linsalata, V. Reggioli. U.O. Fisica Sanitaria,
USL2, Lucca, Italy
Introduction: Due to the higher density housing material of the new de-
tector 729 (PTW) with respect to the old array seven29, it is not
recommended to use the phantom and detector CT to calculate the dose.
PTW suggests to use the CT of the old detector, overriding the density of
the housing material with −300 HU for TPS using AAA algorithm and 1.9 g/
cc for other TPS (for instance tomotherapy). The purpose of this work is
to develop a method to use the new detector 729 without using “artifi-
cial” density on the Octavius phantom CT and to compare the static and
helical calibration of the array.
Methods and materials: We calibrated the detector using a static (G0°
10 × 10 cm2 field) and an helical field (2.5 cm field, pitch 0.287, gantry period
20 sec), comparing the absorbed dose measured by the central chamber
of the array with an ionization chamber put in the same effective point of
measurements in a Solid Water slab or in the Octavius 3D phantom. We
calculated different verification plans using: (1) old Octavius detector
seven29 CT with density override (PTWmethod) and static calibration, (2)
old Octavius detector seven29 CT with static calibration, and (3) Octavius
phantom CT (without detector) with helical calibration.
We compare measured and calculated doses for the 3 different methods.
Results: The comparison among calculated and measured doses has shown
that PTW method underestimates systematically the dose. The results of
verification plans in terms of percentage dose difference and local gamma
analysis 3%/3 mmwere respectively −0.4% and 94% for static calibration and
+0.5% and 96% for helical calibration.
Conclusions: The comparison among the methods showed that the over-
ride density method suggested by PTW underestimates systematically the
dose. The combination of the static calibration with old detector CT and
helical calibration method with Octavius phantom CT (without detector)
was comparable, with an average dose difference from measured and cal-
culated doses within 1%.
A.191
CHARACTERIZATION OF A NEW STEREOTACTIC DIODE UNDER
FLATTENING FILTER FREE BEAMS DOWN TO SMALL FIELDS
G. Reggiori P. Mancosu N. Suchowersk F. Lobefalo A. Stravato
S. Tomatis M. Scorsetti a Medical Physics Service of the Department of
Radiation Oncology, Humanitas Research Hospital, Rozzano, MI, Italy; b School
of Physics, The University of Sydney, Australia; c Department of Radiation
Oncology, Chris O’Brien Lifehouse, Camperdown, Australia
Purpose: To characterize a new unshielded silicon diode prototype (Razor,
IBA) for dose measurements in small beams using flattening filter free beams.
Material and Methods: The detector response stability in measured dose,
dose rate, dose per pulse, and dark current as a function of the received
dose was evaluated. The detector response in square fields (0.8–5.0 cm) was
assessed by means of PDD curves, axial beam profiles and output factors.
The performances were compared to that of the previously available SFD
and the PFD detectors.
Results: The Razor short term stability was much improved relative to the
SFD (<±0.1% after 1.2 kGy). The linearity was <±1% (0.05–30 Gy range) and
dose rate dependence was <±0.5% (4–24 Gy/min range). The dose per pulse
dependence was <± 0.8% (0.08–0.21 cGy/pulse range). Larger dark current
with an increase in dose was observed for the Razor (0.0025 pA/Gy) com-
pared to the SFD (0.0002 pA/Gy). This characteristic is attributed to an
increased concentration of the recombination centers. The measured PDDs
agreed to within 1% with those obtained using the PFD. The profile anal-
ysis showed good results as long as a background correction was applied
before each profile acquisition: penumbra differences were below ±0.3 mm
e56 Abstracts/Physica Medica 32 (2016) e1–e70